1. Field of the Invention
The present invention relates to an electrophotographic apparatus such as a copying machine, and more particularly, to an electrophotographic apparatus for recovering paper dust from recording sheets and residual toner particles from the surface of a photoreceptor.
2. Description of the Related Art
In forming an image in a conventional electrophotographic apparatus, a photoreceptor is rotated and charged by a charging unit, an electrostatic latent image is formed by an exposure unit, and a toner image is formed by a developing unit. Subsequently, the toner image on the photoreceptor is transferred to a recording sheet by a transfer unit, and particles of a toner (residual toner particles) remaining on the photoreceptor are removed by a cleaning unit. The toner particles removed by the cleaning unit are abandoned as waste toner particles.
Furnished with the cleaning unit, however, the electrophotographic apparatus of this type is inevitably large-sized. Moreover, it is uneconomical to abandon the toner recovered by the cleaning unit, and it is troublesome to maintain the cleaning unit which entails a toner disposal process. In an electrophotographic apparatus of a type such that the waste toner is stored in a processing unit (incorporating a photoreceptor, developing unit, etc.), the life of the processing unit is restricted by the storage capacity for the waste toner.
In order to solve these problems, therefore, cleanerless electrophotographic apparatuses have been developed which simultaneously perform developing and cleaning processes. In the electrophotographic apparatuses with this arrangement, after a toner image on a photoreceptor is transferred to a recording sheet by a transfer unit, the residual toner on the photoreceptor is diffused by a stationary diffusion unit. Then, the residual toner is recharged by a charging unit, and is attracted and recovered by a developing roller of a developing unit after an exposure process. Thus, both the developing and the residual toner recovery are accomplished in the developing unit.
The cleanerless electrophotographic apparatuses can be classified into two known types. One is a two-component developing system which uses a two-component developing agent.
However, the two-component developing system used in the conventional electrophotographic apparatuses must be provided with a mechanism for controlling the toner concentration. Therefore, the developing unit is inevitably increased in size and weight, and there are restrictions on voltages applied to the charging unit and the developing roller. If the difference in potential between the photoreceptor and the developing roller is too great, particles of a carrier in the developing agent, which is opposite in polarity to the toner, fly, thereby lowering the performance of the photoreceptor and the image quality.
The other type is a non-contact system in which the photoreceptor is not in contact with the developing roller. According to the conventional non-contact electrophotographic apparatus, however, a DC-superposed AC voltage must be applied to the developing roller in order to increase the difference in potential between the photoreceptor and the developing roller, thus requiring expensive high-voltage power supply equipment.
In the developing unit used in this electrophotographic apparatus, moreover, the charge on the toner should be restricted to a low level in order to allow the toner to fly between the developing roller and the photoreceptor in an AC field. In this electrophotographic apparatus, the residual toner on the photoreceptor is recovered by means of the developing roller after it is recharged by the charging unit. In some cases, however, the toner may be overcharged by the charging unit, so that it cannot be recovered by the developing roller.
In order to solve the various problems of the conventional electrophotographic apparatuses described above, there has been developed an electrophotographic apparatus which incorporates a contact-type developing unit using a nonmagnetic one-component developing system. According to this developing system, a developing electrode is located close to an electrostatic latent image on a photoreceptor drum with a recording sheet and a toner between them, and an electrostatic latent image can be developed faithfully. If the residual toner on the photoreceptor is on a charging potential (white potential), therefore, it can be smoothly recovered by the developing roller.
In some cases, however, the electrophotographic apparatus of this type may suffer image deterioration from the following causes. One of the causes is the influence of paper dust. The paper dust, which is produced from the recording sheet as the sheet is transported, along with the residual toner remaining after a transfer process, adheres to the photoreceptor.
Talc in the paper dust is liable to be charged negatively, that is, it has a marked tendency to charge other materials positively. If the talc adhering to the photoreceptor is carried to the developing unit and recovered together with the residual toner by the developing unit, it will positively charge the negatively charged toner. If a developing operation is performed with use of this positively charged toner in the electrophotographic apparatus which incorporates a negatively charged organic photoreceptor adapted for the negatively charged toner, the toner adheres to a non-image portion, thereby producing a positive image or the so-called fogging which results in a lower image quality.
In the case where the developing unit is of a type such that the toner carried on the developing roller by means of a blade is charged by friction, paper dust sometimes may be jammed between the developing roller and the blade, thereby producing striped or low-quality images.
If an electrostatic latent image is formed on the surface of the photoreceptor with the paper dust thereon by means of the exposure unit, the shielding effect of the paper dust inevitably hinders satisfactory exposure. In this case, the toner cannot be allowed fully to adhere to the photoreceptor by means of the developing unit, so that the toner concentration is too low to prevent the formation of negative memories. Although the conventional electrophotographic apparatuses use the stationary diffusion unit to remove some of the infectious paper dust, their ability to remove the paper dust is not very high.
Another cause of the image deterioration is the influence of the residual toner.
The residual toner remaining on the surface of the photoreceptor after the transfer process is recovered by the developing roller of the developing unit. If the quantity of the residual toner is large, however, the residual toner cannot be recovered satisfactorily. Even after the developing process, therefore, the residual toner remains on the non-image portion of the photoreceptor, thereby forming a positive image. Possibly, moreover, the shielding effect of the toner particles may cause insufficient exposure in the exposure process by means of the exposure unit. In this case, the toner cannot be caused fully to adhere to the photoreceptor in the developing process, so that the toner concentration is too low to prevent the formation of negative memories. Although those problems can be solved to some degree by the use of the stationary diffusion unit, the solution is not satisfactory.
The prevailing versions of the conventional electrophotographic apparatuses use a negatively charged photoreceptor. Negative-charge corotron or scorotron chargers used in negative-charge electrophotographic apparatuses produce much ozone. It is feared that ozone lowers the performance of the photoreceptor. Positive-charge corotron or scorotron chargers produce one-tenth as much ozone as the negative-charge versions. Recently developed electrophotographic apparatuses use a positive-charge photoreceptor in consideration of the aforementioned influences of paper dust and ozone.